1. Field of the Invention
This invention relates to a multi-phase dc motor and particularly relates to a multi-phase dc motor suitable for a hard disc system, a floppy disc system, or a video tape recorder.
2. Description of the Prior Art
Prior art multi-phase dc motors are designed with a given relation between the number of field magnetic poles P and the numbers of slots N. For example, in a three-phase dc motor of a prior art, there are relations as follows: EQU P=(3.+-.1)n EQU N=3n
If the motor is of a type with a core, a peak torque of cogging occurs when a border b between two neighboring poles confronts a slot. Therefore, a frequency of cogging per one rotation is determined by a least common multiple between N and P as shown in TABLE 1 and 2.
Hereinbelow will be described cogging developed in the prior art motor, for example, a motor having a structure shown in TABLE 1 with reference to FIGS. 7 and 8.
FIG. 7 is a cross-sectional view partially shown of a prior art multi-phase dc motor, that is, a three-phase eight-pole twelve-slot dc motor. TABLE 1 shows frequency of cogging with respect to n, N, and P when P=(3-1)n where n is a natural number, N is the number of slots, and P is the number of field magnetic poles. TABLE 2 shows frequency of cogging with respect to n, N, and P when P=(3+1)n.
TABLE 1 ______________________________________ n N P FREQ OF COGGING ______________________________________ 1 3 2 6 2 6 4 12 3 9 6 18 4 12 8 24 5 15 10 30 6 18 12 36 -- -- -- ______________________________________
This prior art three-phase dc motor 3 comprises a core B having 12 slots Q and 12 coils L, a field magnet portion M having eight permanent magnetic poles as a circular array, these permanent magnetic poles being arranged at angularly equally spaced intervals around the core B, N and S poles being arranged alternately, and a yoke Y provided around the field magnet portion M. In this type of three-phase dc motor 3, either of the field magnet portion M or the core B is fixed and the other is rotatable.
TABLE 2 ______________________________________ n N P FREQ OF COGGING ______________________________________ 1 3 4 12 2 6 8 24 3 9 12 36 4 12 16 48 5 15 20 60 6 18 24 72 -- -- -- ______________________________________
In such three-phase dc motor 3, when either of the core B or the field magnet portion M is fixed and the other is rotated, cogging occurs. A peak torque of cogging occurs when a border between two neighboring poles confronts a slot. One protruding (salient) pole provided between two successive slots receives a cogging torque as the rotation between the core B and the field magnet portion M as shown by a waveform 101 in FIG. 8. FIG. 8 shows changes of cogging torque of the prior art motor 3. There are eight 12 slots in the field magnet portion M. Respective protruding poles receive cogging torques as shown by waveforms 101-108. Therefore, a total cogging torque is developed between the core B and the field magnet portion M as shown by waveform 109. There are 24 occurrences of coggings per one rotation. That is, this frequency is the least common multiple between the number of the magnet poles (P=8) and the number of slots (N=12).
In the prior art three-phase dc motor 3, the number of protruding poles where coggings occur at the same time is given by: EQU k=N.times.P/(the least common multiple between N and P)
Therefore, a magnitude of the total cogging torque becomes large. For example, in the above-mentioned three-phase eight-pole twelve-slot dc motor 3, the total cogging torque T is given by: EQU T=kt=12.times.8t/24=4t
wherein t is a magnitude of cogging torque developed at one border g. Therefore, the total cogging torque is four times cogging torque developed at one border g. As a result, there is a problem that jitter, noise, or vibration occurs when such a three-phase dc motor 3 is used as a drum motor or a capstan motor of a video tape recorder.
In order to resolve this problem, there are various techniques disclosed, for example, Japanese patent publication Nos. 62-123944, 64-19949, 63-294234, and 63-316648. However, each of these documents only discloses a motor whose number of slots is m times the number of phases .phi. wherein m is natural number. That is, there is still a problem that jitter, noise, or vibration, occurs when a multi-phase dc motor whose number of slots is not m times the number of phases .phi. is used as a drum motor or a capstan motor of a video tape recorder.